The invention relates to a spectrometer.
The trend in modern spectrometer systems is towards compact and economical detectors affording fast measuring times and a moderate spectral resolution of 5 to 20 nm. Typical applications for such sensors are in the areas of colour measuring techniques or industrial process control.
Spectrometers based on the known principle of diode arrays have the potential to fulfill the above mentioned requirements and form the basis for the present invention. The conditions for the applicability of the invention are discussed below.
The diode array affords a parallel assessment of the entire measured spectrum leading to fast measuring times.
The required spectral resolution is realized with a compact spectrometer geometry with a focal length of less than 30 mm. Several examples are demonstrated with classical spectrometer arrangements based on planar gratings, e.g. a demultiplexer of the Ebert-Fastie geometry as described in U.S. Pat. No. 4,744,618, or concave gratings with an aberration correction, e.g. EP-A 0 489 286.
An economical production requires the use of modern production and assembly techniques. The cost for such spectrometers is mainly determined by three factors: the cost of the components of the diode array and the diffraction grating as well as the cost for the fine tuning of the optical system.
Diode arrays are offered commercially by several manufacturers either in a ceramic or in a plastic housing, for example by Hamamatsu. This type of detector manufacture is used in existing diode array spectrometers, e.g. EP-A 0 489 286. Nevertheless, this detector geometry has several fundamental disadvantages.
First, a quality housing made from ceramic is very expensive and contributes largely to the total cost of a detector unit. Second, for the present application the position of the light sensitive detector area is not sufficiently specified relative to the outer dimension of a standard housing, particularly in a vertical direction. For this reason, the housing cannot serve as a mechanical reference during assembly. The diode array inside the housing, i.e. the actual detector chip or the diffraction grating, has to be actively positioned during assembly on the spectrometer (U.S. Pat. No. 5,040,889, EP-A 0 727 681, EP-A 0 489 286). This manufacturing procedure is costly and associated with high additional costs.
Classical gratings are replicated in an epoxy layer on an optical glass substrate. The procedure is demanding and is not suited for an economical production in large quantities. The costs for the grating can be dramatically reduced by using plastic gratings that are produced in an injection moulding process. This technology has recently become available. The injection moulding process allows for the direct fastening of very precise mechanical positioning aids on the grating component affording a simple assembly of the grating at the central spectrometer module without active adjustment.
The main problem with the use of plastic gratings is however their great temperature sensitivity. The lines of the grating expand proportionally with the thermal expansion coefficient of the plastic material which causes a shift of the measured spectrum with respect to the diode line and introduces an error in the calibration of the wavelength of the spectrometer. Since optical injection moulded plastic materials have thermal expansion coefficients in the order of magnitude of 8xc3x9710xe2x88x925/xc2x0 K, approximately 10 times larger than glass, this spectral shift becomes the main component in the temperature drift of the spectrometer. Thus, a compensation for the temperature drift is a prerequisite for the application of this technology.
Existing techniques for the compensation of temperature drifts of diode array spectrometers are disclosed in documents EP-A 0727 681 and U.S. Pat. No. 4,709,989. The techniques are based on the use of materials having a low thermal expansion coefficient and an exact adaptation of the thermal expansion coefficient of the various spectrometer components. U.S. Pat. No. 4,709,989 relates to the adaptation of the thermal expansion coefficient of the spectral module to the materials of the imaging optics through the use of a ceramic material. EP-A 727 681 discloses an adaptation of the expansion coefficient of the spectral module made of ceramics to the diode array made of silica. Both techniques attempt to yield an approximately even thermal expansion of the entire spectrometer such that the angles of the optical path inside the spectrometer remain the same and the thermal drift is minimized. However, both techniques are restricted in the choice of material for the spectral module and hence can not be used for economically producing spectral sensors from plastic materials.
Thus, there is a need to overcome this difficulty and to improve a spectrometer such that it is very easily and economically assembled, e.g. from components made of plastic materials, to yield, even with the use of plastic components, a small thermal drift sufficient for practical applications.
A spectrometer of the prior art is described in U.S. Pat. No. 5,040,889. In this prior art spectrometer, the detector is arranged on a base board which is moveable relative to the spectrometer housing (carrier) and adjustable by means of a micrometer screw. After successful adjustment, the base board is glued in place. The manufacture of this spectrometer is relatively construction intensive and labour intensive and hence requires great additional costs. Further, the temperature is continuously measured in these prior art spectrometers and an automatic re-calibration is carried out at predetermined temperature changes and at certain time intervals. Hence, only the temperature profile of the sensitivities of the single cells of the detector and their signal amplifiers can be compensated for but not the temperature dependent shift of the measured spectrum with respect to the diode array and thus the resulting error in the wavelength calibration of the spectrometer. The last mentioned problem is not discussed whatsoever in U.S. Pat. No. 5,040,889.
The solution of the object underlying this invention is discussed in the characterizing portion of the independent claim. Preferred embodiments in accordance with the present invention are the subject of the dependent claims.
The new spectrometer geometry in accordance with the spectrometer of the present invention does not require a housing for the detector, particularly the diode array, and is economically produced. Further, the particular spectrometer geometry allows a simple installation at the carrier.
Independently therefrom, mounting the detector on a base board in accordance with the present invention, especially in combination with an adaptation of the choice of material for the carrier, the diffraction grating, and the base board with respect to their expansion coefficients, allows for a complete compensation of the temperature drift in spectrometer systems having components made from plastic materials. The compensation in accordance with the present invention is based on an optimum geometric arrangement and connection of the carrier and the detector. This new technique provides a free choice of material and allows for the consideration of additional technical parameters of the production, such as the manufacturing costs.
In accordance with the invention, the base board is mounted to a carrier in such a way that a temperature dependent shift of a measured spectrum with respect to a detector is at least approximately compensated for, the temperature dependent shift being caused by a thermal expansion of the base board.